How upgrade all weapons?
I would like to know also!
Found a solution from " StrangeInStereo on psnprofiles.
It keeps track of what weapons you’ve upgraded. You can upgrade a few guns and then load your save file. Do that for all the weapons to get the trophy.
There are 14 weapons:
Boomstick
Elysium - LR678
Mimir - PC98
Mimir Needle Pistol
Rambino
Scattergun
Pipegun
State Switcher
Stinger
Crossbow
Gaper
GJR-666
Ancient Pistol
Slinger